For attaining larger capacity in optical transmissions, multicore fibers constructed such as to surround a plurality of cores integrally with a cladding region have been known.
For example, the multicore fiber disclosed in Non-Patent Document 1 can attain low crosstalk when the center-to-center distance of cores is 30 μm, since the power transfer ratio between cores adjacent to each other can be made sufficiently low if the relative refractive index difference Δ of cores with respect to the cladding (hereinafter referred to as core Δ) is changed slightly (e.g., by 0.05%) between the adjacent cores. This is said to be able to achieve a multicore optical fiber having a cladding diameter of 125 μm and including three kinds of cores with different values of core Δ.